The central band of the Milky Way as seen from the ISS. Earth is visible on top. Credit: Scott Kelly/NASA

It is difficult to overstate the significance of light pollution. Light reveals, but if it gets too bright, it blinds – and there’s a derisive irony in there. By the light of stars, the first humans worked day and night to beat back the darkness around them. But today, more than a third of humanity can’t see the Milky Way at night because of artificial lighting, while at least 80% of all people “live under light-polluted skies”.

The findings are the conclusions of a survey undertaken by German, Israeli, Italian and American scientists. Their paper was published in the journal Science Advances on June 10. Its lead author, Fabio Falchi, of the Light Pollution Science and Technology Institute, Thiene, Italy, led the first effort to draw up the artificial brightness atlas a decade ago. The paper finishes thus (with a reference whose relevance is getting harder to miss): “It is possible to imagine two scenarios for the future. Perhaps the current generation will be the final generation to experience such a light-polluted world, as light pollution is successfully controlled. Alternatively, perhaps the world will continue to brighten, with nearly the entire population never experiencing a view of the stars, as in Isaac Asimov’s Nightfall novel and short story.”

A stronger appeal might be to our environmental sense. Humans are diurnal creatures, able to function during both day and night. However, researchers have found that animals, birds and insects can be affected in a variety of ways by unnatural light and brightness. A 2005 book addressing these issues, Ecological Consequences of Artificial Night Lighting, ends thus: “… let us be reminded, as we light the world to suit our needs and whims, that doing so may come at the expense of other living beings, some of whom detect subtle gradations of light to which we are blind, and for whom the night is home.”

The authors of the survey used a combination of data collected by satellites and computer modelling to estimate the amount of light from artificial lighting that was turned upward, into the night sky. They were assisted in particular by sensors installed on the Suomi National Polar-orbiting Partnership (NPP) satellite, operated by the US National Oceanic and Atmospheric Administration (NOAA). Suomi NPP orbits Earth at 824 km in a Sun-synchronous orbit. Its VIIRS DNB sensor measures brightness such that each pixel in its images is 742 meters long, considered to be good spatial resolution. Though some of the readings – as well as the modelling – were confounded by forest fires, volcanoes, lightning, the presence of snow (which reflects light) and aurorae, the authors were assisted by scientists at the NOAA in filtering the data accordingly.

One result of their efforts is this sobering view:

Asia in the New World Atlas of Artificial Sky Brightness, as seen in Google Earth. Source: Science Advances

The Americas in the New World Atlas of Artificial Sky Brightness, as seen in Google Earth. Source: Science Advances

The Middle East and North Africa in the New World Atlas of Artificial Sky Brightness, as seen in Google Earth. Source: Science Advances

The skies over the areas in light blue are 20% more polluted than normal, where the pollution distorts our proper vision of the horizon. Places where this effect extends to the zenith are marked in green. In areas in yellow (around 200%), the Milky Way is not visible in winter nights, when it is typically fainter; in areas in orange, our galaxy isn’t visible in summer nights either. In the areas in red, “people never experience conditions resembling a true night because it is masked by an artificial twilight”. In places marked in white, human eyes are at risk of not being able to adapt to the dark.

On the one hand, light holds a special place in many human cultures, and the ability to manifest it is widely considered a symbol of progress. As the writer Jane Brox put it in 2011, “Before the advent of artificial light, humans had to go to bed when the sun went down.” The presence of street-lamps is thought to make the road or space it lights safer (although the jury’s still out on whether this is always the case). But on the other – as virtually all of India, central Europe and the North American east exemplify – our inefficient use of artificial lighting is blinding us to our stars over growing ranges of urbanisation. It’s easy for some to make this decision but it’s not the one we need to make quickly.

Instead, that involves the use – or disuse – of lighting that doesn’t cast light upward, smart-bulbs that become less bright when not in use (an opportunity Indian entrepreneurs have been encouraged to bank on) and, in general, not installing more power than is required. The authors of the study also recommend reducing the use of “‘blue’ light that interferes with circadian rhythms and scotopic vision”. In some places in India, however, the request could be far simpler: toward a sense of economy, and discouraging the use of artificial lighting as decoration. When political parties had been campaigning ahead of the recently-concluded Tamil Nadu assembly elections, sights like that in the image below were common throughout the state.

A street in West Mambalam, Chennai, decked up with tubelights while a political party was campaigning in the area. Credit: Ananthakrishnan Gopalakrishnan

The Science Advances study also notes some places that are the most removed from the beauty of a pristine sky. This list includes two of the world’s megacities, Bonn-to-Dortmund and Boston-to-Washington, and parts of northern Egypt, England, China and Spain. Something odd for trivia-hunters: “The land on Earth that is farthest from a pristine sky is in Azores [an archipelago in the mid-Atlantic], more than 1,750 km away from the pristine sky of the western Sahara.”

Cloud cover over or near cities and atmospheric pollutants can also increase the amount of reflected light, called skyglow. As human urban centres burgeon and occupy more land, so will the effects on the environment burgeon with them.

In terms of having the most citizens who are affected by worse light pollution, Singapore leads the list: its entire population is in the ‘white’; however, it is also tiny. Four members in the Gulf Countries Council (Kuwait, Qatar, UAE and Saudi Arabia) have more than 80% of their population in similar straits. Saudi also leads in terms of being the most polluted of the G20 countries, followed by South Korea, Argentina, Canada and Spain. India is last on the list: 80% of Indians can at least see the Milky Way on summer nights.

By far the continent with the most countries with pristine dark skies at night is Africa: more than 75% of the population of Chad, the Central African Republic and Madagascar – and more than 98% of the territories of Somalia and Mauritania – experience it. However, this status isn’t the outcome of good planning as much as low electrification. By 2015, only 32.7% of Somalia’s population had access to electricity; the other countries fared worse: Mauritania, 21.8%; Madagascar, 15.4%; Central African Republic, 10.8%; and Chad, 6.4%. And light pollution likely won’t be a big deal for someone angling for 100% from the PoV of 6.4%.

The authors of the study are careful to note the effect their assumptions – such as of atmospheric transparency, the hour of Suomi NPP’s observations, oil-well- and gas-flares, haze, etc. – will have on their findings. However, barring atmospheric transparency, the other factors aren’t widespread, leaving us with a study in contrasts. Astronomers seek out plots of land over which light pollution is ideally absent to better see the dimmer light of distant stars. Yet, there now comes a growing shadow that will hide from us the closer ones.

How bright is the world at night?

Legend: pristine sky; degraded near the horizon; degraded to the zenith; natural sky lost; Milky Way lost; people can’t adapt to night vision.